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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

282

SILVER ALLOYS

at each end. At the upper end is fixed, by means of a rubber stopper, a
glass basin to catch the overflow, while the lower end is connected by a
rubber tube and tap with the vessel containing the sodium chloride solution
fixed at a convenient height. The pipette is filled by opening the tap and
allowing the liquid to flow gently over. When the liquid begins to overflow
at the top, the latter is closed by means of the index finger of the left hand,
while the tap is shut and the rubber tube carefully detached with the left
hand. The lower end is then touched outside with a dry vessel to remove
the small amount of adherent liquid and the bottle placed centrally under

A

V

FIG. 24

FIG. 25

the pipette. The finger is then withdrawn from the top and all the liquid
flowing in a continuous jet, but not the drops falling subsequently, allowed
to run into the bottle.

The pipettes should be kept perfectly free from grease and, before use,
should be washed at least twice with the solution to be measured.

3.  A shaking apparatus, which may be one of those commonly employed
in chemical laboratories for bottles.   In assayers' laboratories special closed
forms of apparatus are used to protect the bottles from the action of the
light.   They take 10 bottles at a time and are often, as in the Mint at Rome,
worked electrically.   The shaking should be rapid and vigorous.

4.  A kind of tray with cells for carrying 10 bottles, screened from the
light, from one part to another of the laboratory.

5.  A water-bath for heating the bottles during the attack of the metal.
Assaying laboratories have also a suitable bench fitted with a back and

a raised ledge placed against a window, facing north if possible. On the
bench the sodium chloride solution is measured, while the solutions which
have already cleared are arranged on the ledge to receive the weaker standardthe causes of error in the deter-